(1) Field of the Invention
The present invention relates to electrostatic chucks of the type in which the temperature of a wafer is to be controlled by transferring heat from plasma or the like to the wafer, thereby making uniform the temperature distribution of the wafer.
(2) Related Art Statement
In the electrostatic chuck, a number of projections or embossed portions are ordinarily projected from an installation surface of an insulator layer, and top surfaces (contact surfaces) of the projections are contacted with a semiconductor wafer. DC voltage is applied to an internal electrode within the insulating layer, and Johnson-Rahbeck force is generated at contact interfaces between the semiconductor wafer and the contact surfaces of the projections to attract the semiconductor wafer on the contact surfaces.
Owing to this, the attraction force for the semiconductor wafer can be enhanced by increasing the area of the contact surfaces (top surfaces) of the projections.
In the thermal CVD or etching of the semiconductor wafer, high density plasma is generated above the semiconductor wafer. In etching, the semiconductor wafer is attracted by the electrostatic chuck, and a cooling flange is provided at a lower side of the electrostatic chuck. Rise in temperature of the semiconductor wafer is prevented by releasing the heat inputted to the semiconductor wafer from the high density plasma to the electrostatic chuck. In the thermal CVD, the temperature of the semi-conductor wafer is controlled to a desired temperature by releasing the heat, which is inputted from the high density plasma to the semiconductor wafer, to the electrostatic chuck from the semiconductor wafer at a constant rate.
However, such a system for controlling the temperature of the semiconductor wafer has the following problem. That is, if heat is inputted to the semiconductor wafer from the high density plasma in the above electrostatic chuck, it is generally difficult to control the degree of the heat conduction from the semiconductor wafer to the electrostatic chuck.
In order to enhance the attracting force for the semiconductor wafer, the area of the contact surfaces of the projections needs to be increased. However, increases in the occupying ratio of the projections causes the following problem.
First, the contact state between the projections and the semi-conductor wafer changes depending upon slight variations in hardness of the contact surfaces of the projections, changes in uneven surfaces, etc., so that the heat contact resistance at the contact surface of each of the projections varies. For this reason, since heat cannot be stably released from the semiconductor wafer to the electrostatic chuck, the uniformity in the temperature of the semiconductor wafer is likely to be deteriorated.
In addition, since the contact heat conduction occurs between the semiconductor wafer and the projections, heat easily transfers between them. Therefore, if the area of the contact surfaces of the projections is increased and the temperature of the semiconductor wafer attempts to be controlled particularly to 100xc2x0 C. or more, and further within a temperature range of 300xc2x0 C. to 400xc2x0 C., for example, the temperature of the semiconductor wafer largely decreases through the contact heat conduction form the semiconductor wafer to the projections. Consequently, the temperature of the semi-conductor wafer cannot be increased in this case.
It is considered that this problem is solved by reducing the heat transmitted from the semiconductor wafer to the electrostatic chuck via the projections through decreasing the area of the contact surface of each of the projections or through reducing the number of projections. However, in this case, the area of the portions of the semiconductor that contact the projections decreases, and the temperature distribution of the semiconductor wafer largely varies, because the amount of heat conducted through heat radiation from the semiconductor wafer to the installation surface of the electrostatic chuck is very small.
Further, this problem is to be solved by flowing a backside gas through a gap between the rear surface of the semiconductor wafer and the insulating layer under a constant pressure and transferring the heat of the semiconductor wafer to the insulating layer through heat conduction with the backside gas. In this method, the heat inputted to the semiconductor wafer transfers into the electrostatic chuck through both the contact heat conduction via contacting between the contact surfaces of the projections and the wafer and the heat conduction with the backside gas. Thereby, variations in the temperature distribution of the semiconductor wafer must be lessened.
However, if the area of the contact surfaces of the projections decreases, the Johnson-Rahbeck force acting between the contact surfaces and the semiconductor wafer decreases. Consequently, the electrostatically attracting force for the semiconductor wafer decreases.
On the other hand, when the backside gas is flown between the rear surface of the semiconductor wafer and the insulating layer under constant pressure, buoyancy acts upon the semiconductor wafer with the backside gas. Owing to this, the attracting force actually acting upon the semiconductor wafer is a value obtained by subtracting the buoyancy upon the semi-conductor wafer with the backside gas from the electrostatically attracting force acting upon the wafer from the electrostatic chuck. If the area of the contact surfaces of the projections is reduced as mentioned above, the function of the buoyancy becomes relatively large, so that the attracting force for the semiconductor wafer becomes insufficient. If the pressure of the backside gas is reduced to avoid such a problem, the heat conduction with the backside gas is insufficient to deteriorate the uniformity of the temperature of the semiconductor wafer.
It is an object of the present invention to provide an electrostatic chuck in which a backside gas is flown in the state that a wafer is attracted to the chuck, heat is inputted to the wafer and the heat of the wafer is transferred to the electrostatic chuck via projections and the backside gas and which facilitates the temperature controlling, and particularly the temperature controlling in a high temperature range for the wafer, and enhances the uniformity of the temperature of the wafer.
The present invention relates to the electrostatic chuck comprising a chuck body, an insulating layer formed on a surface of the chuck body and having an installation surface on which a wafer is to be installed, an inner electrode installed inside the insulating layer, and projections projecting from said installation surface and having contact surfaces to which the wafer is to contact, wherein a back side gas is flown into a space defined by said installation surface, said projections and said wafer, heat is supplied to the wafer, and heat of the wafer is conducted to the electrostatic chuck through the projections and the backside gas, the total area of the contact surfaces of the projections is not more than 1 % of the area of the inner electrode, and the height of the projections are not less than 1 xcexcm and not more than 10 xcexcm.
The present inventors largely reduced the total area of those contact surfaces of the projections which contacted the wafer to not more than 1%, thus largely reduced the ratio of the heat conduction through the contact heat conduction, and thereby facilitated the temperature controlling, particularly the temperature controlling of the semiconductor wafer.
The inventors also discovered that even if the ratio of the contact surfaces of the projections is considerably reduced like this, the heat conduction is effectively effected from the semiconductor wafer to the electrostatic chuck with the backside gas and the uniformity of the temperature of the semiconductor wafer is highly maintained by controlling the height of the projections to not less than 1 xcexcm and not more than 10 xcexcm.
This is further explained. In the prior art, the height of the projections of the electrostatic chuck is around 15 to 50 xcexcm, and heat is transferred between the insulating layer and the semiconductor wafer through heat convection of the gas. Therefore, it is considered disadvantageous to lower the height of the projections from the point of view of the heat conduction.
However, it was clarified that actually controlling the height of the projections to 1 to 10 xcexcm is advantageous for Cheat conduction from another point of view. That is, it seems that a Coulomb force acts between charges located near the surface of the insulating layer and the charges at the semiconductor wafer in addition to the attracting force originating from the Johnson-Rahbeck force at the contact area between the projections and the semiconductor wafer, and it was clarified that the electrostatically attracting force for the semiconductor wafer is not lowered beyond expectation. As a result, the present inventors succeeded in making the temperature distribution of the semiconductor wafer uniform by increasing the pressure of the backside gas between the rear surface the semiconductor wafer and the installation surface of the insulating layer and effectively perform heat conduction with the backside gas. To attain the above function and effects, it is necessary to set the height of the projections to not more than 10 xcexcm. From the above point of view, the height of the projections is preferably not more than 8 xcexcm.
On the other hand, it is clarified that as the height of the projections decreases, the contributory rate of the above Coulomb force increases to further enhance the electrostatically attracting force. However, if the height of the projections is less than 1 Em, the wafer is attracted to the chuck at portions other than the projections. Therefore, the height of the projections needs to be not less than 1 xcexcm. If the height of the projections is less than 5 xcexcm, the backside gas is unlikely to be distributed over the entire rear surface of the wafer even under elevated pressure of the backside gas, so that the efficiency of the heat conduction decreases, and the uniformity of the temperature of the semiconductor wafer deteriorates. Probably, this is considered to the effect that if the height of the projections is less than 5 xcexcm, the heat convection has no contribution, and the heat radiation becomes predominant. From this point of view, the height of the projections is preferably not less than 5 xcexcm.
From the point of view of further suppressing the contact heat conduction through the projections, the total area of the contact surfaces of the projections is preferably not more than 0.9 %, more preferably not more than 0.6 % of the area of the inner electrode.
From the point of view of stably supporting and attracting the wafer, the total area of the contact surfaces of the projections is preferably not less than 0.2 %, more preferably not less than 0.4 % of the area of the inner electrode.
The area of those contact surfaces of the projections which contact the wafer means an area of those contact surfaces of the projections which contact the rear surface of the wafer in ordinary attracting. This area is ordinarily equal to the total area of the top surfaces of the projections. However, if a part of the projections is lower and does not contact the rear surface of the wafer in the ordinary installation condition, for example, the area of the top surfaces of the parts of the projections is not included in the above total area.
The area of the inner electrode and that of the contact surface of the projection are both measured in a vertical direction to the installation surface.
The height of the projection is measured with a dial gauge or a three-dimensional shape measurement device.
Heat is inputted to the semiconductor with plasma, particularly preferably with die high density plasma, but heat radiation may be employed for this purpose.
These and other features, advantages will be appreciated with the understanding that some modifications, changes and variations could be made by the skilled person in the art to which pertains.